Method and apparatus for communication in ultra-wide bandwidth rfid systems

ABSTRACT

A method for communication in ultra-wide bandwidth RFID systems includes the steps of generating through a transceiver device an interrogation signal composed of at least a sequence of equally time spaced pulses within a period and transmitting the signal to at least one RFID identification device which generates a response signal and transmits it back to the transceiver device. The interrogation signal is coded according to a numerical coding sequence. The response signal is generated by varying, in each period of the pulse sequence of the interrogation signal, a polarity of pulses using the numerical coding sequence. An apparatus for communication in ultra-wide bandwidth RFID systems includes a transceiver device and a RFID identification device, suitable for communicating with each other. The RFID identification device includes a backscatter modulator suitable for modulating a signal reflected by an antenna of an identification device RFID.

TECHNICAL FIELD

The invention relates to a method of producing phase-pure nano plate lithium metal (II) phosphate crystals and a lithium metal (II) phosphate crystal material.

BACKGROUND AND RELATED ART

Batteries belong to the most important power sources which are used in different areas of operation. Almost any electrical consumer can be equipped with batteries in order to use electrical energy which results from discharging of the battery through an electrochemical redox reaction.

Lithium is a widespread negative electrode material for batteries. This is due to the fact that lithium has the most negative standard potential of all elements which allows realizing obtaining high battery cell voltages. Also, using lithium theoretically extremely high battery capacities are accomplishable. Indeed, since many years suitable electrode materials are developed for uptaking and releasing of lithium ions in combination with respective electrolyte materials in order to achieve such high theoretical energy densities of lithium batteries in practice. One electrode material which can be used to realize such high discharge voltages while maintaining a high capacity are lithium phosphor compounds in the form of olivines, as for example LiMPO₄, wherein M is a metal like iron, manganese, cobalt etc.

Phospho-olivines such as LiMPO₄ (M=transition metal) are attractive candidates for cathode materials in lithium ion batteries, because of their excellent cyclability, thermal stability, low cost and environmental benefits. However, these materials suffer from a low electronic and ionic conductivity of LiMPO₄ leading typically to poor rate capabilities. Immense technical efforts have therefore been devoted to counteract this problem, one approach being the synthesis of well dispersed and small particles to shorten the diffusion path length of lithium ions.

For example, J. Electrochem. Soc., Vol. 144, No. 4, April 1997, p. 1188-1194 discloses that phospho-olivine is a positive electrode material suitable for rechargeable lithium batteries.

Similarly, J. Electrochem. Soc., Vol. 148, No. 8, A960-A967, 2001 deals with the usage of olivine type lithium compounds as a possible cathode material for lithium batteries.

U.S. Pat. No. 5,910,382 discloses the usage of transition metal compounds with an ordered olivine or rhombohedral Nasicon structure as electrode material for rechargeable alkali ion batteries.

The electrochemical behaviour of olivine nanoparticles (LiMnPO₄) prepared via a polyol route with particles sizes between 50 and 150 nm was studied by T. R. Kim, D. H. Kim, H. W. Ryu, J. H. Moon, J. H. Lee, S. Boo, J. Kim, in J. Phys. Chem. Solids 2007, Vol. 68, 1203-1206, and found to be good. In another publication by H. Ehrenberg and N. N. Bramnik, the influence of a decreasing particle size (down to 100 nm) on the low rate capability of LiMnPO₄ cathode materials was studied (J. Alloy Comp, 2007). However, no report on the production of plate-like nanocrystals with a thickness of the plates below 20 nm is given.

SUMMARY OF THE INVENTION

The present invention provides a method of producing phase-pure nano plate lithium metal (II) phosphate crystals, the method comprising mixing a lithium salt, a transition metal salt, a phosphor source and a phosphate terminated surfactant and heat treating the mixture.

In accordance with an embodiment of the invention, the phosphate terminated surfactant is a phosphate terminated surfactant which preferably comprises an alkyl aryl alkoxy phosphate salt. In accordance with a further embodiment of the invention, the phase-pure nano plate lithium metal (II) phosphate crystals are phosphor-olivine crystals. Examples for alkyl aryl alkoxy phosphate salts are for example Triton H-66, H-33, H-155 and BMP100. These are surfactant materials which are anionic hydrotropes. Other examples for phosphate-terminated surfactants include TRP 40, LMP 30, or NOR 90.

The nano plate lithium metal (II) phosphate crystals have the advantage, that a lithium ion mobility is drastically improved due to the plate like structure with a typical thickness of the plates around 1 nm to 20 nm, preferably below 5 nm. Thus, the diffusion length of lithium ions is reduced. As already mentioned above, a high lithium ion mobility corresponds to high charge and discharge rates which is an important aspect for usage of LiMPO₄ in lithium ion batteries.

By adding the phosphate terminated surfactant, the lithium metal (II) phosphate crystal surface is covered by the phosphate terminated surfactant such that the crystal growth of olivine crystals is strongly suppressed in z-axis so that a plate like lithium metal (II) phosphate crystal is obtained. As a consequence, due to the extremely thin lithium metal (II) phosphate crystals, the specific surface area of the crystal material is tremendously increased such that the lithium ion mobility through the surface is additionally improved.

In accordance with an embodiment of the invention, the method further comprises adding a pH adjusting chemical for adjusting the pH value of the mixture to a predetermined value. Preferably, the pH adjusting chemical is added last in the sequence of adding the above mentioned lithium salt, transition metal salt, phosphor source and phosphate terminated surfactant. Preferably, the pH value is kept by means of the pH adjusting chemical in the range between 9 and 11. This helps avoiding an unwanted precipitation of solids.

In accordance with an embodiment of the invention, the method further comprises washing and/or drying the lithium metal (II) phosphate crystals. By washing the lithium metal (II) phosphate crystals, unwanted residual surfactant is removed which allows to obtain pure lithium metal (II) phosphate crystals which further increases the lithium ion mobility.

In accordance with a further embodiment of the invention, the method further comprises dissolving the phosphate terminated surfactant in water and heating the resulting solution, wherein the lithium salt, transition metal salt and phosphor source are added to the heated solution.

In accordance with an embodiment of the invention, the heating of the mixture is performed by a hydrothermal treatment and/or ambient pressure treatment and/or microwave treatment. However, a hydrothermal treatment is preferred since this drastically speeds up the reaction kinetics such that also in a commercially applicable timeframe a production of said phase-pure nano plate lithium metal (II) phosphate crystals is possible.

In accordance with an embodiment of the invention, the nano plate lithium metal (II) phosphate crystals contain material with the composition Li_(x)M_(y)PO₄ with M=Ti, V, W, Cr, Mn, Fe, Co, Ni, Cu, Mg, Ca, Sr, Pb, Cd, Ba, Be, and/or contain material of the composition Li_(x)Fe_(1-y)Ti_(y)PO₄ and/or Li_(x)Fe_(1-y)Mn_(y)PO₄ with 0<y<1.

In accordance with an embodiment of the invention, the phosphor source comprises a phosphate and/or a phosphoric acid component.

In accordance with a further embodiment of the invention, the pH adjusting component comprises LiOH.

In accordance with a further embodiment of the invention, the method further comprises adding a cationic surfactant, wherein the cationic surfactant is dissolved together with the phosphate terminated surfactant in water. Preferably, the cationic surfactant comprises cetyltrimethylammoniumbromide (CTAB).

The usage of a mixture of alkyl aryl alkoxy phosphates with a cationic surfactant has the advantage, that the surface area of the produced nano plate lithium metal (II) phosphate crystals is further increased. As already mentioned above, a larger surface of the plate like crystals corresponds to an increased lithium ion mobility which thus further increases the charge/discharge rates of lithium ion batteries comprising such kinds of lithium metal (II) phosphate, preferably phospho-olivines as electrode material.

In another aspect, the invention relates to a lithium metal (II) phosphate crystal material, wherein the material comprises a nano plate crystal structure, wherein the crystal structure is phase-pure.

In accordance with an embodiment of the invention, the crystal thickness is below 20 nm, preferably below 5 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the invention are described in greater detail by way of example only making reference to the drawings in which:

FIG. 1 is a TEM (transmission electron microscope) photograph showing nano plate LIMnPO₄, the insert on the left bottom side displaying the side view on the plates.

FIG. 2 is a flowchart illustrating the method of producing phase-pure nano plate phospho-olivine crystals according to the invention,

FIG. 3 is a schematic illustrating the suppression of crystal growth in the z-axis of LiMnPO₄,

FIG. 4 shows an X-ray diffractogram (XRD) of phase-pure, crystalline LiMnPO₄ according to the invention,

FIG. 5 illustrates different measurement results for the specific surface area (SSA) for LiMnPO₄ prepared with a variety of surfactants.

DETAILED DESCRIPTION

FIG. 1 shows a TEM photograph showing nano plate LiMnPO₄ with a specific surface area greater than 50 m² per gram. It can be clearly seen, that in the top view (large image) the LiMnPO₄ crystals have a rectangular type structure, with a width in between 50 and 100 nm. The typical crystal length is about 200-400 nm.

The insert TEM image in FIG. 1 illustrates the thickness of the individual plates. As can be clearly seen in the insert of FIG. 1, individual crystals 102 and 104 are stacked on top of each other, with a thickness of around 5 nm. In general, the TEM images show that the resulting LiMnPO₄ consists of crystalline layers and particles. The surface area increases, for the more pronounced layered structures. Particle sizes of LiMnPO₄ crystals decrease from several 100 nm without surfactant to 5 nm by using a mixture of cetyltrimethylammonium bromide (CTAB) and Triton H-66 as surfactants.

FIG. 2 is a flowchart illustrating the method of producing phase-pure nano plate phospho-olivine crystals according to the invention. In step 200, solutions are prepared in which a alkyl aryl alkoxy phosphate salt as well as optionally a further surfactant like CTAB are brought into an aqueous solution. Preferably, a weight ratio between water and the surfactants is around 80-20.

In step 202, the solution of the surfactants is heated up to 100° C., during which water was outgased by an argon flow. This is followed by step 204, in which the lithium salt, and transition metal salt and a phosphor source are added and mixed by steering with the solutions of the surfactants.

In order to for example synthesize lithium manganese phosphate, as transition metal salt Mn(NO₃)₂×4H₂O, as lithium salt LiNO₃×2H₂O and as phosphor source H₃PO₄ can be used.

After all substances were added and dissolved, after step 204, step 206 is carried out in which a pH adjusting component is slowly added to the solution. For example, the pH adjusting component is LiOH.

After that, in step 208, the reaction solution is put for example into an autoclave and heated up to 150° C. for 36-48 hours. This leads to a precipitation of resulting LiMnPO₄ which in step 210 is filtered and washed with ethanol and acetone. Preferably, after step 210 a further drying step is carried out which ensures that the resulting LiMnPO₄ is free of solvents and water.

FIG. 3 is a schematic illustrating the suppression of LiMnPO₄ crystal growth in the z-axis, i.e. perpendicularly to the main extension directions of the LiMnPO₄ crystal. The LiMnPO₄ crystal 300 illustrated in FIG. 3 has a length of about 300 nm and a thickness of about 5 nm. As can be seen in FIG. 3, the phosphate groups 304 of the alkyl aryl alkoxy phosphate components 302 are coordinated to the manganese ions of LiMnPO₄. This means, that in the direction of the manganese-phosphate coupling, a crystal growth of LiMnPO₄ 300 in the extension direction of said complexation, which is said z-direction, is blocked, such that only a crystal growth in directions 306 and 308 perpendicularly to the z-direction is possible.

FIG. 4 shows an X-ray diffraction pattern of LiMnPO₄ obtained according to this invention. The measurement was taken on a Panalytical EXPERT Pro diffractometer usind CuKα radiation in a scanning range from 10° to 70° 2 Theta. The diagram clearly shows, that the synthesized LiMnPO₄ has a phase-pure, crystalline structure. No further phosphate impurities are detectable, which, as already mentioned above, helps to further increase the lithium ion mobility for example in lithium ion batteries applications.

FIG. 5 illustrates a comparison of different results of a specific surface area measured for nano plate phospho-olivine crystals synthesized with different surfactants. In sample 500, no surfactant, i.e. no alkyl aryl alkoxy phosphate salt, was used for producing the phospho-olivine crystals. This resulted in a specific surface area SSA below 10 m² per gram. Also adding of sodium dodecyl sulfate (SOS), which is an anionic surfactant in sample 502 or adding of Pluronic P123, which is a non-ionic surfactant does not lead to an increase of the specific surface area. The same holds with respect to sample 506 in which only CTAB as cationic surfactant was used.

In contrast, in samples 508 and 510, the alkyl aryl alkoxy phosphate salt was used for producing the phospho-olivine crystals. For example, by using Triton H-66 as surfactant a significantly higher surface area up to 40 m² per gram can be obtained. The specific surface area can further be increased by additionally adding CTAB to the alkyl aryl alkoxy phosphate salt. In this case, the specific surface area can even be increased to above 70 m² per gram. Such high specific surface area values are highly desirable for usage of the phospho-olivine crystal material in high power rechargeable lithium based batteries.

LIST OF REFERENCE NUMERALS

-   -   100 top view of olivine crystal     -   102 side view of olivine crystal     -   104 side view of olivine crystal     -   300 phospho olivine crystal     -   302 alkyl aryl alkoxy phosphate component     -   304 phosphor group     -   306 direction     -   308 direction     -   500 component     -   502 component     -   504 component     -   506 component     -   508 component     -   510 component 

1-20. (canceled)
 21. A method for communication in ultra-wide bandwidth RFID systems comprising the steps of generating through a transceiver device an interrogation ultra-wide bandwidth signal, said signal comprising at least a sequence of equally time spaced pulses within a period; transmitting said signal to at least one RFID identification device; processing said interrogation signal according to information stored in said RFID device and transmitting it back to said transceiver device as a response signal; receiving said response signal through said transceiver device and further processing said response signal to detect said information included in said response signal; coding said interrogation signal according to a numerical coding sequence that determines a polarity of each pulse of said sequence of pulses; and generating said response signal in said RFID identification device by varying through a variation of an antenna load, in each period of said sequence of pulses of said interrogation signal, a polarity of pulses using said numerical coding sequence, so that all reflected pulses in a same period have a same polarity.
 22. A method according to claim 21, wherein said numerical sequence is a binary c_(n)ε{−1,1}, or ternary c_(n)ε{−1,0,1} numerical sequence.
 23. A method according to claim 22, wherein said numerical sequence is a zero or quasi-zero sum numerical sequence.
 24. A method according to claim 23, wherein in each period of said numerical sequence a number of positive units is equal to the number of negative units.
 25. A method according to claim 23, wherein in each period of said numerical sequence a number of positive units differs from the number of negative units in one unit.
 26. A method according to claim 21, further comprising summing together said pulses in each period of a response signal.
 27. A method according to claim 21, wherein a duration of each pulse in said sequence of pulses is less than one nanosecond.
 28. A method according to claim 21, wherein a time interval between two successive pulses of said sequence of pulses is not less than 50 nanoseconds.
 29. A method according to claim 21, wherein a time interval between two successive pulses of said sequence of pulses is greater than 100 nanoseconds.
 30. A method for communication in ultra-wide bandwidth RFID systems comprising the steps of generating through a transceiver device an interrogation ultra-wide bandwidth signal, said signal comprising at least a sequence of equally time spaced pulses within a period; transmitting said signal to at least one RFID identification device; processing said interrogation signal according to information stored in said RFID device and transmitting it back to a transceiver device as a response signal; receiving said response signal through said transceiver device and further processing said signal to detect said information included in said response signal; generating said sequence of pulses so that all pulses of said sequence have a same polarity; generating said response signal in said RFID identification device by coding said interrogation signal according to a numerical coding sequence and to said information stored in said RFID device that determines a polarity of each pulse of said sequence of pulses by varying antenna load conditions; and decoding, in said transceiver device, said response signal by modifying, in each period of said sequence of pulses of a response signal, a polarity of pulses using said numerical coding sequence, so that all pulses within a same period have a same polarity.
 31. A method according to claim 30, wherein said numerical sequence is a binary C_(n)ε{−1,1}, or ternary c_(n)ε{−1,0,1} numerical sequence.
 32. A method according to claim 31, wherein said numerical sequence is a zero or quasi-zero sum numerical sequence.
 33. A method according to claim 32, wherein in each period of said numerical sequence a number of positive units is equal to the number of negative units.
 34. A method according to claim 32, wherein in each period of said numerical sequence a number of positive units differs from the number of negative units in one unit.
 35. A method according to claim 30, further comprising summing together said pulses in each period of a response signal after said decoding.
 36. A method according to claim 30, wherein a duration of each pulse in said sequence of pulses is less than one nanosecond.
 37. A method according to claim 30, wherein a time interval between two successive pulses of said sequence of pulses is not less than 50 nanoseconds.
 38. A method according to claim 30, wherein a time interval between two successive pulses of said sequence of pulses is greater than 100 nanoseconds.
 39. An apparatus for communication in ultra-wide bandwidth RFID systems, comprising a transceiver device and a RFID identification device suitable for communicating each other, wherein said RFID identification device comprises a backscatter modulator suitable for modulating a signal generated by said transceiver device and reflected back by an antenna of a RFID identification device, said backscatter modulator comprising a first switch placed in parallel to an antenna and a second switch, parallel to an antenna as well, and in series to a load impedance.
 40. An apparatus according to claim 39, wherein said backscatter modulator further comprises a delay line placed in series to an antenna and comprising a third switch and a fourth switch.
 41. An apparatus according to claim 39, wherein said transceiver device comprises a transmitting portion and a receiving portion, said transmitting portion comprising a first numerical sequence generator operationally associated with a pulse generator.
 42. An apparatus according to claim 39, wherein said transceiver device comprises a transmitting portion and a receiving portion, said transmitting portion comprising a pulse generator and said receiving portion comprising a first numerical sequence generator associated with a multiplier device suitable for multiplying a signal received by said receiving portion by said numerical sequence.
 43. An apparatus according to claim 41, wherein said first numerical sequence generator generates a binary c_(n)ε{−1,1}, or ternary c_(n)ε{−1,0,1} numerical sequence.
 44. An apparatus according to claim 42, wherein said first numerical sequence generator generates a binary c_(n)ε{−1,1}, or ternary c_(n)ε{−1,0,1} numerical sequence.
 45. An apparatus according to claim 41, wherein said receiving portion comprises a matched filter, a sampler device, an accumulation unit, a threshold detector, an antenna and a switch to connect alternatively said antenna to said transmitting portion and said receiving portion.
 46. An apparatus according to claim 42, wherein said receiving portion comprises a matched filter, a sampler device, an accumulation unit, a threshold detector, an antenna and a switch to connect alternatively said antenna to said transmitting portion and said receiving portion.
 47. An apparatus according to claim 39, wherein said RFID identification device comprises a second numerical sequence generator, a control unit, a synchronization unit, and a memory unit.
 48. An apparatus according to claim 47, wherein said second numerical sequence generator generates a numerical sequence identical to a numerical sequence generated by said first numerical sequence generator.
 49. An apparatus according to claim 47, wherein said RFID identification device further comprises a power unit. 